This invention relates to drilling devices, and more particularly, to asymmetrical drills.
In human and veterinary medicine, it is often necessary to remove or cut live bone, such as in the drilling of holes into teeth in dentistry, the cutting of a jawbone in oral surgery, the cutting of the skull in neurosurgery, etc. In such cases it is highly desirable, and in many cases absolutely necessary, that the surrounding bone not be damaged. This protection of the surrounding bone is particularly necessary to promote the healing of bone about implants inserted in the holes.
Further, damage to such bone can be expected at relatively low temperatures, e.g. teeth or other bony structure will only tolerate temperatures of approximately 50.degree. C. All this makes it important to reduce the amount of heat generated during drilling.
It is known to dissipate frictional heat by conducting cooling fluids, such as water and physiological saline solution, etc., as close as possible to the place of heat formation.
A known type of prior art device dissipates frictional heat by employing nozzles to discharge cooling fluid on the drill bit. In dentistry, these nozzles are usually arranged on a dental handpiece for a drill. The nozzles spray the cooling fluid onto the drilling site. However, this method can not be used when drilling is done in relatively deep holes or if mechanical obstacles prevent the access of the cooling fluid to the tool portion which is in engagement with the substance to be removed.
U.S. Pat. No. 3,762,052 to Melde provides a drill with a longitudinal channel passing through the entire cutting element and which opens outwardly at the working end of the tool. However, the cutting elements of such drills becomes so greatly weakened after hollowing out of the passages that great difficulties of manufacture arise. Moreover, it is technologically difficult and time consuming to make such passages in very compact cutting elements of manufactured from relatively hard material.
In many instances a substantial amount of heat is generated during drilling, as a result of close engagement between a cutting element of a drill and a bone. This often occurs when a symmetrical or concentric cutting member of a drill has a longitudinal axis which is concentric with the longitudinal axis of the drill shank. Such a concentric cutting drill is described by U.S. Pat. No. 4,021,920 to Kirschner et al. During operation of the concentric drill of Kirschner et al, the shank and the cutting element rotate about the same longitudinal axis of rotation. As a result, the opening to be drilled is concentric with the longitudinal axis or the axis of rotation of the cutting element. Therefore, a substantial part of the outside surface of the cutting element engages a significant part of the surface of the bone into which it is drilled. The larger the area of engagement between the bone and the cutting element, the greater the friction and the amount of heat which is generated during the drilling. In the concentric drills such area of engagement and friction is close to the maximum.
When the concentric drill has an irrigation channel which opens at the working end of the drill (see the patent to Melde, for example), the cutting element blocks the irrigation fluid in the hole and prevents the fluid from leaving it. This is because of the substantial engagement between the concentric cutting element and the hole in the bone. As a result, the irrigational fluid blocked in the bone hole can itself reach quite high temperatures and does not effectively reduce the amount of heat generated during the drilling.
Moreover, the symmetrical cutting element, by closing the bone crypt during the process of drilling, prevents the discharge of the cut debris out of the hole or crypt. Thus, in order to facilitate the flushing away of the cut debris, repeated removal of the drill from the crypt and additional irrigation is needed.